The present invention relates generally to an analysis system for use with an alternating current (AC) electric utility power system, and more particularly to an arc spectral shape analysis system for detecting high impedance, low current arcing faults on the power system. Arcing faults may be caused by, for example, downed, broken, tangled or dangling power lines, trees contacting the power lines, and various overcurrent fault situations.
Arcing faults are more difficult to detect than permanent overcurrent faults, which for instance, occur when a transformer fails. Conventional overcurrent protection devices have time delays which permit a temporary fault from de-energizing the power line. Only if the overcurrent fault persists does such a protection device de-energize the power line. Some arcing faults may initialize the timing circuits of the overcurrent protection devices but, by the end of the delay, the high impedance nature of the fault limits the fault current to a low value. An overcurrent protection device thus cannot distinguish this low fault current from the levels of current ordinarily drawn by customers, so the lines may remain energized even though a dangerous arcing fault exists on the power line.
Other methods of detecting arcing faults have focused on the high harmonic frequency content of the line current. These earlier methods compared the magnitude of line current harmonics with a predetermined reference magnitude. For instance, U.S. Pat. No. 3,308,345 to Warrington detects arcing faults by first filtering out the fundamental frequency (e.g. 60 Hertz in the United States and 50 Hertz in Europe) and its second and third harmonics. The magnitude of the remaining high harmonic frequencies, i.e., the fourth, fifth . . . harmonic frequencies, are then compared to a predetermined threshold magnitude value. Warrington measures the signals over a predetermined length of time. If the magnitude value of the high harmonic frequency components exceeds the reference magnitude value, the Warrington device produces a warning signal. The magnitude of normal harmonic components varies significantly depending on the connected load, making it extremely difficult to effectively set the threshold magnitude value for the Warrington device.
However, arcing often produces burst energy at low frequencies, particularly at non-harmonic frequencies near the fundamental frequency. The earlier methods failed to analyze the non-harmonic frequencies. Also, if the reference magnitude values were set too low, the earlier detection systems would often be too sensitive. As a result, the power lines would be de-energized although no hazardous fault existed on the line. Similarly, if the reference magnitude values were set too high, the lines would remain energized even though a dangerous fault existed on the power line.
Thus, a need exists for an improved high impedance fault detection system for electrical power transmission and distribution systems which is directed toward overcoming, and not susceptible to, these limitations and disadvantages.